Small Computer System Interface (SCSI)
(SCSI, pronounced "SKUH-zee") (n.) An independent standard for a system-level interface between computers and such peripherals as hard disks, CD-ROMs, printers, and scanners. SCSI can connect a number of devices to a single controller on the computerís bus. All SCSI chains require termination at both ends, and devices on a SCSI chain must be set to different ID numbers. SCSI connections normally use single-ended drivers, as opposed to differential drivers. Single-ended SCSI can support up to six meters of cable, whereas differential SCSI can support up to 25 meters. Apple Computer established the SCSI interface in 1984 on the Macintosh computer. It was developed by Shugart Associates, which later became Seagate, so SCSI was originally called Shugart Associates System Interface (SASI) before it became an ANSI standard in 1986. The original standard is now called SCSI-1 to distinguish it from SCSI-2 and SCSI-3, which include specifications for wide SCSI and fast SCSI, described in the paragraphs below.

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SCSI-1: Small Computer System Interface version 1; an eight-bit-wide parallel bus used to connect disk drives and all types of peripherals to an IBM-compatible PC or to a Macintosh. The origins of SCSI are found in the selector channel architecture designed by IBM for its System/360 computers. In 1981, Shugart Associates adapted it and called it the Shugart Associates Systems Interface (SASI). By 1984 Apple Computer had incorporated it in the Macintosh. In 1986, ANSI released the first version of the SCSI standard. That original version is now called SCSI-1.

SCSI supports hard disk drives, CD-ROM drives, optical storage units, scanners, high-speed printers, and Group 4 fax machines. Up to eight devices can share the bus, including the host controller. Each device on the bus is manually assigned a SCSI ID number between 0 and 7 by means of a switch or jumper. The number of devices supported per bus is limited by the width of the data bus. A SCSI-1 chain may include eight devices, whereas a 16-bit-wide SCSI-2 or SCSI-3 bus may support up to 16 devices.

The SCSI adapter in the host PC is normally assigned SCSI ID 7, because the highest number wins a bus arbitration. Peripherals are usually assigned IDs starting at 0 Each end of a SCSI bus must be terminated with a resistor. Other devices on the SCSI bus must not be terminated.

Macintosh computers and peripherals are connected by either DB-25 or DB-50 SCSI connectors. IBM-compatible PCs use the RJ-21 Centronics-style 50-pin connector for SCSI-1 devices. To manage peripherals on a SCSI bus, Advanced SCSI Programming Interface (ASPI) software is used in the IBM PC environment. SCSI supports multiple host adapters on each bus, so that two computers may be connected to the same SCSI bus and can control the same peripheral.

Types of SCSI
Connectors

Connector
End

P=male;
S=female

Description

RC-50 P/S

full-pitch
Centronic 50-pin

DB-25 P/S

full-pitch
D-Sub 25-pin

DB-50 P/S

full-pitch
D-Sub 50-pin

RCII-50 P/S

Centronic
half-pitch micro connector 50-pin

DBII-50 P/S

D-Sub
half-pitch micro connector 50-pin

RC3-68 P/S

Centronic
half-pitch micro connector 68-pin

DB3-68 P/S

D-Sub
half-pitch micro connector 68-pin

SCSI data transfers may be either asynchronous or synchronous. In the asynchronous mode there is a handshake for each transferred byte, and the maximum data transfer rate is about two megabits per second (Mbps). In the synchronous mode data flows at a preset data rate, which is commonly five Mbps for SCSI-1 devices. The initiator and the target peripheral must support synchronous transfers at the same speed.

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SCSI-2: Small Computer System Interface version 2; enhancement to the SCSI-1 specification that delivers more compatibility among devices and faster data transfer rates. Fast SCSI is a subset of SCSI-2 and supports transfer rates of up to 10 megabits per second (Mbps), double the five-Mbps rate of SCSI-1 on an eight-bit-wide bus. The SCSI-2 specification defines five device types not covered by SCSI-1.

The original eight-bit-wide SCSI-1 bus is called narrow SCSI. The connectors for the 16-bit-wide bus differs from the SCSI-1 connector. A cable with 50-pin Micro-D-type connectors is specified for 16-bit transfers. For the 32-bit bus, or wide SCSI, a 68-pin P-type connector is specified. The SCSI-2 bus requires an odd parity bit for every eight data bits, whereas parity was optional in SCSI-1.

A fast, wide SCSI bus can support data transfer at up to 40 Mbps, although actual transfer rates from a disk drive are typically much lower, depending on the speed of the drive. SCSI-2 controllers support different speeds of transfer to each SCSI device on the bus, so both SCSI-1 and SCSI-2 devices can coexist. An extension of SCSI-2 called ultra SCSI doubles the transfer speed of fast SCSI to allow transfer rates of 20 Mbps on an eight-bit connection and 40 Mbps on a 16-bit connection.

The following is a comparison of maximum theoretical data transfer rates for hard drive interfaces. Actual throughput is much slower than these maximum rates, depending on drive access times and other factors in each computer system.

Disk Interface Maximum

Transfer

Rate

SCSI-1 (asynchronous) 2 Mbps

SCSI-1 (synchronous) 5 Mbps

SCSI-2 fast 10 Mbps

SCSI-2 16-bit-wide 10 Mbps

SCSI-2 fast and 16-bit-wide 20 Mbps

SCSI-2 32-bit-wide 20 Mbps

SCSI-2 fast and 32-bit-wide 40 Mbps

When SCSI-2 and SCSI-3 devices first connect asynchronously, they handshake in eight-bit narrow mode to ensure that data will transfer properly. The initiator and target then negotiate the maximum mutual capabilities.

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SCSI-3: Small Computer System Interface version 3; enhancement to SCSI-2 that supports up to 32 devices per bus with 16-bit transfers over a P-cable (using a 68-pin Micro-D-type connector). A subset of SCSI-3 called fast-20 supports the transfer of data at up to 20 megabits per second (Mbps) over the original eight-bit "narrow" bus and up to 40 Mbps over a 16-bit-wide P-cable. Fast-20 controllers allow both fast and slow devices on the same bus. Because of the higher transfer rate, cables should not exceed 3 meters in length for single-ended SCSI buses, about half the distance allowed in SCSI-2 connections. For a differential bus the maximum length is 25 meters, which is the same as for SCSI-2. SCSI-2 has largely replaced SCSI-1, but SCSI-3 is not as prevalent.

SCSI-3 features a serial option using a six-pin connector. Serial SCSI supports various types of media, including fiber and twisted pairs for distances of up to a kilometer. The serial version offers a structured protocol model, faster data transfer, longer cables, and more device classes. It increases to 32 the number of devices that may share a bus. Embedding clock information in the serial data stream eliminates signal delay problems. SCSI-3 is a combination of separate standards defined by separate groups, as listed in the SCSI-3 table.